In an optical network, such as a Passive Optical access Network, each user is provided with their own optical apparatus. Information is sent to each user's optical apparatus from a piece of equipment on the network such as an optical line terminal OLT. The OLT may be connected to each user's apparatus through a single fibre and an optical distribution node, which is typically a passive splitter in a conventional Passive optical network (PON) or a wavelength selective splitter in a wavelength division multiplexing (WDM) PON. The role of the user's optical apparatus is to send an optical signal—which the user can modulate with data—across the network for transmission to other equipment on the network. It will typically send that information to a central office. The user apparatus may also function to read optical information sent to it from the network. This enables the user to send and receive data optically across the network. In the case of an International Telecommunications (ITU) standard WDM network the user's optical apparatus may form part of an item of equipment that is sometimes referred to as an optical network terminal (ONT).
Wavelength division multiplexing is a very promising technology for optical communications networks due to the effective use of the network fibres. Relatively high upstream and downstream bandwidth can be provided compared with time division multiplexing. Wavelength multiplexing allows a single fibre to exit the central office carrying a plurality of different wavelength optical signals with the signals being subsequently split by a wavelength selective splitter for transmission along 4, 8, 16, 32, 40, 64 or 80 fibres, which are each connected to a respective item of user optical apparatus near the end of the communication path. In a WDM PON, different users are associated with different wavelengths.
Several standards exist for WDM transport networks, an example of a standard for a WDM-PON transport network being the 50 GHz ITU-T grid standard. Data carrying signals are transmitted upstream and downstream by the central office by modulating a set of continuous wave carrier signals which are spaced at intervals of 50 GHz. Each signal is allocated uniquely to a user's optical apparatus.
A WDM PON requires each user to transmit information to the network on a different wavelength. Unfortunately, this means that, historically, the optical apparatus provided for each user at their optical network termination has to be slightly different from that of each other user to enable the apparatus to transmit a signal at its unique allocated wavelength. In practice this means that each piece of user optical apparatus has to include unique and/or complex componentry, making production costs high. In the case of access networks, and in particular passive optical networks, in which a single fibre is used to carry data to each user, cost is a key driver.